The present invention relates to the art of diagnostic imaging. It finds particular application in conjunction with spiral volume imaging with CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the invention will also find application in conjunction with other types of volume imaging of human patients for medical diagnostic purposes, of manufactured articles to detect internal structures or flaws, and the like.
In conventional spiral or helical scanning, an x-ray tube rotates continuously as the subject support table moves at a constant, linear velocity. In this manner, the collected data effectively represents a helical path of constant pitch through the subject. In some spiral scanners, a second spiral scan is collected in either the same direction after retrace or in the opposite direction during retrace. Retrace is the return of the subject support to the initial starting position.
Rather than collecting the second helical path of data after the retrace, the data along the second or larger plurality of helical paths can be collected concurrently with the first helix. Multiple rings of detectors, as illustrated in the parent applications hereto, enable data to be collected along two or more helical paths concurrently.
Commonly, the image data are stored and handled as a series of parallel planar images or slices transverse to the longitudinal axis of the subject. More specifically, the collection of planar slices is typically treated as a three dimensional rectangular matrix of image information. That is, the data collected along the helical paths is reconstructed into a series of planar slices.
One technique for reconstructing image slices from spiral data is to use interpolation or weighting to convert the spiral data to planar data. For example, for each plane, the corresponding rays in the two closest spirals to either side of the plane are interpolated to generate one of the rays of the planar data set. This same interpolating procedure is repeated for each ray of a complete data set, e.g. a 180.degree. plus fan beam data set. This data can then be reconstructed into a planar image representation using conventional reconstruction techniques.
In order to optimize the resolution of the reconstructed data and resultant images, it would be advantageous to minimize the pitch of the helixes, i.e. minimize longitudinal movement of the subject support per revolution. However, the pitch is often constrained by the selected size of the imaged volume and the selected duration within which the data is to be collected. For human subjects, particularly in areas where organs are moving, the volume scan must be conducted sufficiently quickly that image degradation from organ movement is minimized. Once the image scan time and the size of the volume are fixed, the pitch is constrained.
Another factor for optimizing the resolution is the spacing between the adjacent detector rings. At first blush, it might appear that minimizing the detector spacing would optimize resolution. The inventors herein have determined that minimizing the detector spacing in most cases does not optimize the resolution.
Previously, dual slice spiral CT scanning was considered to aid only in x-ray photon utilization. Using two interleaved spirals does not necessarily generate higher resolution data. Much of the data is redundant or partially redundant, i.e. taken along time displaced fully or partially overlapping rays. The redundant rays are combined to double the x-ray photon utilization, but the redundant rays do not in general double the sampling.
The present application contemplates a new and improved multiple helix scanning technique which overcomes the above-referenced problems and others.